1. Field of the Invention
The invention relates to a semiconductor wafer having a back surface and a front surface which has been coated by chemical vapor deposition (CVD) and a polished or etched back surface, and to a process for producing the semiconductor wafer. The invention also relates to a susceptor for a semiconductor wafer to be placed on during the deposition of a layer on a front surface of the semiconductor wafer by chemical vapor deposition (CVD).
2. Background Art
Chemical vapor deposition (CVD), in particular deposition of an epitaxial layer on a silicon substrate wafer which has been polished on both sides, involves, inter alia, confronting two phenomena known as “autodoping” and “halo”. In the case of autodoping, dopants pass from the back surface of the semiconductor wafer via the vapor phase into the deposition gas which is passed over the front surface of the semiconductor wafer. They are then incorporated in the epitaxial layer predominantly in the edge region of the front surface of the semiconductor wafer and as a result cause a more or less pronounced undesirable radial fluctuation in the resistivity of the epitaxial layer.
The term halo denotes a scattered-light effect which is caused by light-scattering structures on the back surface of the semiconductor wafer and manifests itself when the back surface of the semiconductor wafer is exposed to a focused light beam. The structures mark transitions on the back surface of the semiconductor wafer at which regions with a native oxide layer adjoin regions where an oxide layer of this type is not present. The transitions, which are likewise undesirable, occur if the removal of the native oxide layer during a preheating phase known as the “prebake”, prior to the actual deposition phase, was incomplete. One way of quantifying the halo effect is a scattered-light measurement of the haze, for example using a Tencor SP1 appliance in what is known as the DNN (DarkField Narrow Normal) or DWN (DarkField Wide Normal) channel.
To avoid problems with autodoping, it is proposed in U.S. Pat. No. 6,129,047 to provide slots in the base of the pocket in the susceptor which receives the semiconductor wafer, the slots being arranged at the outer edge of the base. Dopants which diffuse out from the back surface of the semiconductor wafer can be removed from the reactor by purge gas which is passed through slots in the susceptor onto the back surface of the wafer without reaching the front surface of the semiconductor wafer. According to US 2001/0037761 A1, small holes are provided throughout the entire base of the susceptor for the same purpose. In this case too, the dopant which diffuses out from the back surface of the semiconductor wafer is carried away by the passage of a purge gas. The measures are also effective at preventing halo formation, since they facilitate the removal of the native oxide layer, since gaseous reaction products which are formed during the dissolution of the native oxide are likewise transported away through the holes in the base and the purge gas flowing past.
However, the use of the susceptor described is not altogether without problems, since the holes influence the temperature field on the back surface and front surface of the semiconductor wafer. If the diameter of the holes in the base of the susceptor exceeds a certain size, this has an adverse effect on the nanotopography of the front surface of the semiconductor wafer. The term nanotopography is used to describe height fluctuations in the nanometer range, measured over a lateral range from 0.5 to 10 mm. The temperature fluctuations caused by the holes lead to locally different deposition rates during deposition of the epitaxial layer on the front surface of the semiconductor wafer and ultimately to the abovementioned height fluctuations. To avoid this problem, in US-2001/0037761 A1, it is proposed that the diameter of the holes be restricted and that the temperature field be made more even by adapting the power of the lamp heating. However, these measures are only effective with regard to the front surface of the semiconductor wafer. As the inventors of the present invention have discovered, the nanotopography values on the back surface of the semiconductor wafer become even worse with decreasing diameter of the holes in the susceptor. On account of the presence of the holes, the temperature field on the back surface of the semiconductor wafer remains so uneven that local etching caused by purge gas, for example hydrogen, and local deposition, caused by deposition gas reaching the back surface of the semiconductor wafer, both occur. Each have a deleterious effect on the nanotopography of the back surface, which cannot be tolerated since even unevenness on the back surface of the semiconductor wafer may cause focusing problems for the stepper during the production of electronic components on the front surface of the semiconductor wafer.